The present invention relates to an apparatus and method for coating printed sheets.
In recent years, high speed digital printing systems have been introduced with the capacity to produce high quality images on sheets of various sizes and at various sheet rates. Such a system is exemplified by the Xerox DucuColor iGen3™ digital production press. This system is capable of handling multiple sheet sizes and can produce up to 6000 impressions per hour or more. A need exists for providing high quality coatings on the printed sheets as they are being produced by such printers, e.g., in order to protect the printed images.
Coating machines are known in which printed sheets are fed through a coating nip formed between a coating cylinder and an impression cylinder. Coating material is distributed to an image area of a coating cylinder, e.g., an image area formed by a coating plate mounted on the coating cylinder, and is applied to the sheets. It is important that the leading edges of the printed sheets be properly synchronized with the printing plate as the sheet enters the coating nip, in order to ensure that the printed image on the sheet is properly coated.
Ideally, the sheets would be sent to the coating apparatus upon exiting the printer. A problem exists, however, because printed sheets do not necessarily exit the printer at a constant cadence, or frequency, thus making it difficult to synchronize the leading edge of the sheet with the image area as the leading edge enters the coating nip.
Even if the sheets were to exit the printer at a constant cadence, the instant at which the sheets exit the printer cannot be predicted, so there may be no opportunity to set the timing of the coating cylinder to conform to sheet arrival.
Therefore, a need exists for a coating apparatus and method which enables sheets received by the coating apparatus, especially received at a non-uniform cadence or at unpredictable moments, to be properly synchronized with the image area of a rotating coating cylinder.
It would also be desirable for the printed sheet to arrive at the printer while traveling at a speed equal to the surface speed of the coating cylinder.
Preferably, it should be possible for such a coating apparatus and method to be adapted to coat sheets of different lengths.
Moreover, it is common, once printed sheets have been coated, to feed the coated side of the sheet beneath a curing device, such as one or more high-wattage ultraviolet or infrared lamps. In the event that a sheet was to become jammed beneath the curing device, heat from the lamps could cause the sheet to reach a temperature high enough to ignite the sheet. Thus, it would be desirable to minimize the chances for coated sheets to ignite.